1. Field of the Invention
The present invention relates, in general, to a II-VI quantum well laser emitting a blue-green light and, more particularly, to a tensile strained blue-green II-VI quantum well laser having an active region consisting of ZnSSe quantum well with tensile strain and a barrier region allowed to lattice match with ZnSe, capable of lowering threshold current densities and of shortening oscillation wavelength.
2. Description of the Prior Art
Even though II-VI semiconductors are attractive for potential optoelectronic devices in a blue-green spectrum, they have been slowly developed in the early time because of the difficulty in growing high-quality heterostructures, in making p-n junctions and in forming ohmic or low-resistivity contacts. The presence of a large number of background impurities and the lack of understanding the role of shallow acceptors and donors also have hindered the progress of the above II-VI semiconductors.
Recent advances in growth technology of II-VI materials, such as organo-metallic vapor phase epitaxy (hereinafter "OMVPE") and molecular beam epitaxy (hereinafter "MBE"), now allow the growing of high-quality heterostructures with reduced background impurities ranging from 10.sup.14 to 10.sup.15 cm.sup.-3 and with doping levels ranging from 10.sup.17 to 10.sup.18 cm.sup.-3, leading to the realization of p-n junction lasers in the blue-green spectrum.
By the way, thethreshold current densities of these II-VI quantum well lasers are believed to be substantially larger than those of GaAs quantum well lasers. This is partially due to the fact that the optical gain of a ZnSe quantum well is much smaller than that of the GaAs quantum well under the same injection condition.
On the other hand, it is well known that biaxially or uniaxially strained quantum well, such as InGaAs-AlGaAs, have enhanced optical gain as compared with unstrained quantum wells, such as GaAs-AlGaAs. This suggests that the unfavorably situation for II-VI quantum well lasers can be overcome by introducing well-engineered biaxial compressive strain. Biaxially compressed II-VI quantum wells can be achieved by choosing the barrier material with a smaller lattice constant than that of the active region.
Description is given next for a conventional blue-green II-VI quantum well laser for better understanding of the background of the present invention, with reference to FIG. 1. As shown in this figure, one of the possible candidates for a biaxially compressed quantum well is a Cd.sub.x Zn.sub.1-x Se-ZnS.sub.y Se.sub.1-y system.
That is, on a p type GaAS substrate 11, there are grown a p type ZnSe layer 12, a p type ZnS.sub.y Se.sub.1-y cladding layer 13, a non-doped cd.sub.x Zn.sub.1-x Se active region 14, which is at most several hundreds .ANG. thick, an n type ZnS.sub.y Se.sub.1-y cladding layer 15, an n type current-restricting layer, an n type ZnSe layer 17 and an n.sup.+ cap layer 18, in due order, where 0.ltoreq.y.ltoreq.0.07 and 0.ltoreq.x.ltoreq.0.25.
If the active region 14 consists of ZnSe in the conventional strained blue-green II-VI quantum well laser having the above structure, the optical gain of the active region quantum well becomes very small.
However, while the addition of Cd to the ZnSe active region in amounts of not more than 25% for making the lattice constant of the active region 14 be larger than that of the cladding layers 13 and 15 allows the optical gain of the active region quantum well to increase as well as applies biaxial compressive strain to the active region, the oscillation wavelength of II-VI laser becomes lengthy in proportion to the amount of Cd added.
Consequently, the conventional strained blue-green II-VI quantum well laser has a limit such that its oscillation wavelength does not become shorter than 500 nm.